Bulletin of the American Physical Society
62nd Annual Meeting of the APS Division of Plasma Physics
Volume 65, Number 11
Monday–Friday, November 9–13, 2020; Remote; Time Zone: Central Standard Time, USA
Session GO03: Astrophysical Plasma: Jets, GRBs, Pulsars, White Dwarfs, and Computational TechniquesLive
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Chair: Sasha Philippov, Flatiron Institute |
Tuesday, November 10, 2020 9:30AM - 9:42AM Live |
GO03.00001: Magnetic Field Decay and Particle Acceleration when Large-amplitude Magnetic Shear Waves Propagating through a Pulsar Wind Termination Shock Yingchao Lu, Fan Guo, Hui Li, Patrick Kilian, Chengkun Huang, Edison Liang How magnetic field energy converts to particle energy is a major problem in highly magnetized outflows in pulsar wind nebulae, gamma-ray bursts and jets from black holes. Here we report PIC plasma kinetic simulations of a large-amplitude, circularly polarized wave passing through a relativistic shock, an expected situation at the pulsar wind termination shock. We find that this process is subject to dissipation of superluminal waves produced by parametric decaying instability at the shock front. This leads to efficient decay of magnetic field and allow strong particle acceleration at the shock front. [Preview Abstract] |
Tuesday, November 10, 2020 9:42AM - 9:54AM Live |
GO03.00002: Non-adiabatic electron heating in supernova remnant shocks Vasileios Tsiolis, Patrick Crumley, Anatoly Spitkovsky We investigate electron heating in collisionless, non-relativistic, perpendicular electron-ion shocks from first principles. We employ numerical, fully kinetic, two-dimensional particle-in-cell simulations to follow the shock formation until the downstream steady-state in electron-ion temperature ratio $T_e/T_i$ is reached. Our simulations are performed in a range of Alfvenic, $M_A$, and sonic Mach numbers, $M_s$, ranging from 2 to 68. We find that for low sonic Mach numbers, the electron-ion temperature ratio shows weak dependence on $M_A$ and the two species are closer to equipartition. At higher $M_s$, the temperature ratio is primarily determined by $M_A$, showing a minimum value of $T_e/T_i \approx 0.1$ at $M_A \approx 10$ and reaching an asymptotic value of $T_e/T_i \approx 0.3$ at higher Mach numbers. At high $M_A$ the shock structure becomes filamentary because of the Weibel instability. The presence of the filaments in density and magnetic field at the shock foot and ramp is responsible for the non-adiabatic heating of electrons, as these filaments provide channels of acceleration along the field lines due to fast electrostatic oscillations associated with the cross-shock potential. The physical picture described here is relevant in supernova remnant shocks. [Preview Abstract] |
Tuesday, November 10, 2020 9:54AM - 10:06AM Live |
GO03.00003: Developing the wave equation for self-consistent metric oscillations in plasma Deepen Garg, I. Y. Dodin Electromagnetic (EM) radiation has been seen to accompany gravitational-wave (GW) bursts from neutron-star mergers. However, the linear coupling between GWs and EM fields is yet to be described self-consistently with sufficient rigor. The standard approach to this problem has been to solve Einstein equations with matter and fields as source terms, but this has proven to be prohibitively cumbersome. We use an alternative, variational formulation [arXiv:2005.01256] to derive the wave equation for collective oscillations of the self-consistent metric with a general polarization. As limiting cases, we reproduce the vacuum GWs as well as the Jeans instability which is the gravitational counterpart of Langmuir waves. Developing further on this equation, we also present corrections to the geometrical optics of GWs, which are of the same order as the GW-matter interaction term for near-vacuum waves. [Preview Abstract] |
Tuesday, November 10, 2020 10:06AM - 10:18AM Live |
GO03.00004: Weak Alfv\'{e}n wave collisions in relativistic plasma turbulence Jason TenBarge, Bart Ripperda, Amitava Bhattacharjee, James Juno, Alexander Chernoglazov, Elias Most, Alexander Philippov It is well established that Alfv\'{e}n waves are the primary building blocks of the non-relativistic turbulence that permeates the heliosphere and low to moderate energy astrophysical systems. However, many astrophysical systems such as gamma-ray bursts, pulsar and magnetar magnetospheres, and active galactic nuclei have relativistic flows or energy densities. To better understand these high energy systems, we examine asymptotically weak Alfv\'{e}nic turbulence through third order in both relativistic magnetohydrodynamics (RMHD) and force-free MHD. We compare both numerical and analytical asymptotic solutions to demonstrate that many of the findings from non-relativistic weak turbulence carry-over to the relativistic system, but an important distinction in the relativistic limit is finite coupling to the compressible fast mode. Since fast modes can propagate across field lines, this mechanism provides a route for energy to escape strongly magnetized systems, e.g., magnetar magnetospheres. [Preview Abstract] |
Tuesday, November 10, 2020 10:18AM - 10:30AM Live |
GO03.00005: Hard Synchrotron Spectra from Magnetically Dominated Plasma Turbulence Luca Comisso, Emanuele Sobacchi, Lorenzo Sironi Synchrotron emission from astrophysical nonthermal sources usually assumes that the emitting particles are isotropic. By means of large-scale two- and three-dimensional particle-in-cell simulations, we demonstrate that the dissipation of magnetically dominated turbulence in pair plasmas leads to strongly anisotropic particle distributions. At modest Lorentz factors, of the order of the plasma magnetization, the particle velocity is preferentially aligned with the local magnetic field. On the other hand, the highest-energy particles are preferentially oriented in the plane perpendicular to the magnetic field. This energy-dependent anisotropy leads to a synchrotron spectral flux that is much harder than for isotropic particles. Remarkably, for large values of the plasma magnetization, the angle-integrated spectral slope is nearly independent of the level of the turbulence fluctuations, despite significant variations in the power-law energy spectrum of nonthermal particles. This is because weaker turbulence levels imprint a stronger degree of anisotropy, thereby counteracting the effect of the steeper particle spectrum. Our findings may help explain the origin of hard synchrotron spectra of astrophysical nonthermal sources, most notably the radio spectrum of pulsar wind nebulae. [Preview Abstract] |
Tuesday, November 10, 2020 10:30AM - 10:42AM Live |
GO03.00006: A New C/O Plasma Melting Curve for White Dwarf Core Crystallization Simon Blouin, Jerome Daligault, Didier Saumon White dwarfs are burned-out stars condemned to a slow cooling that extends over billions of years. Thanks to this simple evolution, it is relatively easy to measure their ages, making them useful cosmic clocks to study the history of our Galaxy. Eventually, the dense C/O plasma that makes up their cores becomes so correlated that it freezes. This process releases latent heat as well as gravitational energy due to the sedimentation of the O-enriched solid. This new energy source temporarily slows the cooling of the white dwarf and it is important to precisely model it if those stars are to be used for precision cosmochronology. Both the melting temperature and the importance of O sedimentation depend on the exact shape of the C/O phase diagram. We present a new C/O phase diagram obtained with the Gibbs-Duhem integration technique and semi-grand canonical Monte-Carlo simulations of the liquid and solid phases of the screened, partially relativistic, fully ionized mixture. This method---applied here for the first time to plasmas---allows us to obtain a definitive version of the classical C/O phase diagram, free of the limitations and approximations of previous calculations. Our results lead to an improved match between white dwarf evolution models and astronomical observations. [Preview Abstract] |
Tuesday, November 10, 2020 10:42AM - 10:54AM Live |
GO03.00007: Fireball beam formation and magnetization via plasma microinstabilities from first principles Bertrand Martinez, Thomas Grismayer, Luis O. Silva Gamma Ray Bursts represent the most intense sources of hard photons ever observed by astronomers. They involve the explosion of a stellar mass object, which energy is expelled in the form of a hot and expanding fireball beam. At large radius, the latter becomes optically thin to hard photons, thus enabling the propagation of gamma ray flashes in the Circum Burst Medium. The self-consistent interaction of these gamma ray bursts with a background plasma is still an unsolved issue, in particular how they transfer energy to it. For the first time, we focus on the self-consistent interaction of an intense gamma ray source with a background pair plasma. We performed first-principles simulations with the code Osiris, recently enriched with a module to account for Compton scattering. We prove that an intense flash of gamma rays travelling in a pair plasma can create a fireball. The latter is self-sustained by the incident gamma rays, and continuously filaments, self-consistently generating small scale B field over the depletion length of the beam. [Preview Abstract] |
Tuesday, November 10, 2020 10:54AM - 11:06AM Live |
GO03.00008: Ab Initio vs Reduced Pair Production Models for Pair Discharges in Pulsar Magnetospheres Fabio Cruz, Thomas Grismayer, Luis O Silva, Alexander Y Chen, Anatoly Spitkovsky Pulsar magnetospheres are thought to be filled with pair plasma generated in strong discharges. The driving mechanism of these discharges is the consecutive emission of gamma-ray curvature radiation, its reabsorption in the extreme magnetic field of these objects, and the subsequent production of pairs via Quantum Electrodynamics (QED) processes. Modelling pair discharges from first principles in this setting is challenging, and has only been possible using one dimensional simulations. However, the field structure in the regions where these discharges develop is intrinsically multi-dimensional. In this work, we present 2D cylindrical particle-in-cell simulations of pair discharges in pulsar polar caps with realistic magnetic field geometry and including the QED processes from first principles. We show that electrostatic plasma waves are generated during the periodic pair discharges, and convert to an electromagnetic mode that is highly collimated along the pulsar magnetic axis. Furthermore, we propose a reduced model for the pair production process that qualitatively reproduces the \textit{ab initio} results. Using this model, we analytically describe the development of the pair cascade, demonstrate that the final plasma configuration is unstable and show that it determines the final amplitude of the generated plasma waves. [Preview Abstract] |
Tuesday, November 10, 2020 11:06AM - 11:18AM Live |
GO03.00009: Theory of plasma wakes driven by Compton scattering Thomas Grismayer, Fabrizio Del Gaudio, Ricardo Fonseca, Luis Silva Photon bursts with a wavelength smaller than the plasma inter-particle distance can drive plasma wakes via Compton scattering [1]. Such wakes are likely to be formed in astrophysical environments where abundant energetic photons are produced. We present here a complete one dimensional theory of this fundamental process, which is compared with the results of PIC simulations enriched with a Compton module [2]. We take into account several parameters such the length of the photon driver, the initial energy density (number density and frequency), and the plasma magnetization. A special focus is also dedicated to the difference with other drivers (laser / particle beam) that excite plasma modes via their effective ponderomotive force. Our results show that Langmuir and extraordinary modes are driven efficiently when the photon energy density lies above a certain threshold. The interaction of photon bursts with magnetized plasmas is of distinguished interest as the generated extraordinary modes can convert into pure electromagnetic waves at the plasma/vacuum boundary. [1] F. Del Gaudio et al. submitted (2020) arXiv:2003.04249 [2] F. Del Gaudio et al. submitted (2020) arXiv:2004.11404 [Preview Abstract] |
Tuesday, November 10, 2020 11:18AM - 11:30AM Live |
GO03.00010: Gamma-ray flares in strongly radiative magnetic reconnection Kevin Schoeffler, Thomas Grismayer, Dmitri Uzdensky, Luis Silva The time evolution of radiation generated in reconnecting relativistic pair plasma is investigated comparing 3D and 2D particle-in-cell simulations in strong magnetic fields. The compression of plasma, magnetic fields, and increased heating at the center of magnetic islands during reconnection leads to a sudden flaring of emission. Although radiative cooling weakens these flares, it also leads to further compression of the islands, which amplifies fields and plasma density (1), and mitigates this effect. Measured increases in density n and magnetic fields B due to compression are visible in n-B space. The quantum electrodynamic (QED) module (2) of the OSIRIS framework models radiation as either classical radiation reaction or QED emission of discrete photons by non-linear Compton scattering, as well as single-photon decay into pairs (non-linear Breit-Wheeler). These QED effects are important for the field strengths close to the critical (Schwinger) field occurring in magnetar magnetospheres, where gamma-ray flares occur. We show that the sudden enhancement of radiation is expected to occur in regions with strong fields, leading to both gamma-ray emission and pair production.\\(1) K. Schoeffler et al., ApJ, 870, 1 (2019)\\(2) T. Grismayer et al., Phys. Plasmas 23, 056706 (2016) [Preview Abstract] |
Tuesday, November 10, 2020 11:30AM - 11:42AM Live |
GO03.00011: Magnetic Field Amplification by a Nonlinear Electron Streaming Instability J. Ryan Peterson, Siegfried Glenzer, Frederico Fiuza Magnetic field amplification by streaming instabilities is central to many astrophysical scenarios, from supernova remnant shocks to gamma-ray bursts. It can also be important in advanced laboratory concepts for inertial fusion and compact radiation sources. The Weibel, or current filamentation, instability is often thought to be the dominant amplification mechanism in weakly magnetized environments, but it operates only at very small (skin depth) scales. We report a new nonlinear electron streaming instability that arises due to the propagation of highly relativistic electrons in a background plasma after saturation of the Weibel instability. This instability gives rise to large cavities in the background plasma that grow radially, leading to exponential amplification of the magnetic field strength and spatial scale over many orders of magnitude. It saturates when the gyroradius of the relativistic electrons becomes comparable to the size of the cavity (Alfvén limit), producing near-equipartition magnetic fields even when the electron beam density is much smaller than the background density. Analytical scalings for the growth rate, wavelength, and saturation amplitude are shown to be in good agreement with multidimensional particle-in-cell simulations. [Preview Abstract] |
Tuesday, November 10, 2020 11:42AM - 11:54AM Live |
GO03.00012: New Thermal Conductivities for Warm Dense Matter and the Age of the Galaxy Nathaniel Shaffer, Simon Blouin, Didier Saumon, Charles Starrett In white dwarfs that undergo convective coupling, the degenerate conductive core and outer convective plasma are separated by a layer of warm dense matter. This layer acts as a bottleneck for the transport of energy from the core to the convection zone, and the cooling rate and inferred age of the white dwarf are sensitive to the thermal conductivity of this warm dense plasma. However, the thermal conduction models used in most stellar evolution codes do not treat these conditions accurately, due to a combination of ion Coulomb correlations, partially degenerate electrons, and e-e scattering. The recently developed mean-force quantum Landau-Fokker-Planck kinetic theory tackles all these challenges and provides an accurate and predictive theory of thermal conduction at conditions relevant to convective coupling in H- and He-dominated white dwarf atmospheres. It is found that the bottom of the convection zone of these white dwarfs can be 2-3 times more conductive than predicted by the widely used model of Cassisi et al. (2007). This leads to more rapid cooling and dimming of the star, on the order of 1 Gyr for massive white dwarfs. Taken in isolation, this result would dramatically impact the use of white dwarfs as cosmological clocks. [Preview Abstract] |
Tuesday, November 10, 2020 11:54AM - 12:06PM Live |
GO03.00013: Modelling Particle Acceleration in Kinetic Simulations of Relativistic Pair Plasma Turbulence Kai Wong, Vladimir Zhdankin, Dmitri Uzdensky, Gregory Werner, Mitchell Begelman Magnetised turbulent astrophysical systems such as pulsar wind nebulae, accretion flows, and jets from active galactic nuclei generate nonthermal populations of relativistic high-energy particles. To understand the physical processes underlying nonthermal particle acceleration (NTPA) in these environments, we study 3D particle-in-cell simulations of driven turbulence in relativistic pair plasma. By tracking large numbers of particles, we obtain statistical measurements of NTPA and compare them to analytical theories. The NTPA can be described by a Fokker-Planck stochastic energy diffusion-advection model with energy-dependent diffusion and advection coefficients $D$ and $A$. We investigate the dependence of $D$ and $A$ on the particle energy $\epsilon$ and on system parameters such as the initial magnetisation, instantaneous magnetisation $\sigma$, system size, and turbulence driving scale. In the nonthermal energy range, we find $D \sim \epsilon^2 \sigma^{3/2}$. We also investigate the time evolution of turbulent fluctuation spectra and the power-law index of the nonthermal particle energy distribution, and their relationship to $D$. These results shed light on the physical mechanisms and theories governing turbulent NTPA. [Preview Abstract] |
Tuesday, November 10, 2020 12:06PM - 12:18PM Live |
GO03.00014: On the possibility of generating ultra-dense fireball pair beams at CERN Charles Arrowsmith, Robert Bingham, Tristan Davenne, Nitin Shukla, Nikos Charitonidis, Yacine Kadi, Todd Huffman, Brian Reville, Scott Richardson, Hui Chen, Luis Silva, Subir Sarkar, Gianluca Gregori Several extreme astrophysical phenomena are observed on Earth as ultra-high energy radiation and cosmic rays, of which the specific generation mechanisms remain an open question. In the case of the fireball model of gamma ray bursts (GRBs), relativistic jets with an arbitrary mixture of electrons-positrons-hadrons are unstable to plasma instabilities in the background medium. Particle-in-cell (PIC) simulations of pair beams propagating in plasmas have confirmed the role of such instabilities in generating strong microscopic magnetic fields relevant to explaining radiation signatures of GRBs. It remains beyond the reach of simulation to explore the long-term evolution of these magnetic fields and address questions surrounding the emergence of coherent macroscopic structures which are key for modelling emission from collisionless shocks. Creating ultra-dense relativistic pair beams is an ongoing experimental challenge. However, recent Monte-Carlo simulations with the FLUKA code modelling 440 GeV protons at the CERN HiRadMat facility show promising results, giving rise to the possibility of generating pair beams with high enough densities for experimental observation of plasma instabilities relevant to GRB fireballs and related afterglow evolution. [Preview Abstract] |
Tuesday, November 10, 2020 12:18PM - 12:30PM On Demand |
GO03.00015: A Machine-Learned Orbital-Free Force-Correction Model: Extending the Thermodynamic Range of Affordable Kohn-Sham Level Accuracy Joshua Hinz, Valentin Karasiev, Suxing Hu Density functional theory (DFT) is an essential tool for material property predictions in warm dense matter. DFT-based molecular-dynamic (MD) predictions require a delicate balance of computational cost and desired accuracy. Orbital-based Mermin--Kohn--Sham (MKS) DFT is a widely successful branch of DFT; unfortunately, the computational cost scales cubically with the number of thermally occupied orbitals; leading to prohibitive costs at higher temperatures. Alternatively, orbital-free (OF)-DFT is orders of magnitude faster but tends to be less accurate than MKS-DFT. In this work we have developed a machine-learning--based force-correction model, using a deep neural network, to map OF-DFT forces to the corresponding MKS forces. With the use of local ion descriptors in conjunction with the less-accurate OF forces, the model is able to significantly reduce the relative error between OF and MKS forces, thereby enabling MD simulations with the best of both worlds. The success of the model is benchmarked by reproducing DFT predictions for the insulator-to-metal transition of fluid hydrogen---a system that is strongly dependent on the underlying ion configurations. The potential strengths and weaknesses of the model will be discussed. [Preview Abstract] |
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